Apple - Do LAN transfers use both Ethernet and WiFi by default?

Do LAN transfers use both Ethernet and WiFi by default?

No. The default action is to use the one with the highest priority. This is usually done by the order of the interfaces that you specify in Network Preferences. Following that, the next order of priority is network latency.

If not, is there a way to enable transfers to use both?

What you are referring to is called link aggregation or bonding. Your Mac is definitely capable of doing this, however, you must connect to a switch (usually a "smart switch") that also has this capability. So, unless your switch has this ability, you can't do it.

Also, you can't bond WiFi and Ethernet; link aggregation is for bonding Ethernet links.

If not, is there a way to enable transfers to use both?

Yes. While it is complicated (or, with cheaper devices, impossible) to do this on the link, network, or routing layers, you can use both Ethernet and Wifi by "bundling" your two links on the application layer easily.

Put both your devices on Ethernet and Wifi; and make sure Ethernet/Wifi are in different subnets. Then split your 100GB of files into two sets, their size roughly corresponding to the relative speed of the two connections.

Then, connect from the iMac to the NAS twice at the same time, once for each of the two IP addresses of the NAS. I have used a Mac one time in my life, about 20 years ago, so I have no idea how you do that, but I am still sure that it is somehow possible (in the worst case, by not mounting the NAS file system directly on the iMac, but by using something like a scp/ftp/rsync file transfer instead.

Then transfer the two sets of files you separated earlier, one to the first IP address, the other to the other. The TCP/IP traffic will go over the respective link only, and assuming that both devices are able to handle that capacity (i.e., the drives are fast enough, no artificial bottleneck due to badly optimized network stacks, etc.), you will get a maximum performance close to the sum of the two bandwiths.